It might be about love. True stories often involve relationships, and 'Best of Me' could center around the different forms of love - romantic, platonic, or self - love - that the characters experience throughout the story.
The main theme could be self - discovery. In many true - story - based works like this, the protagonist often goes through a journey of finding out who they truly are, facing their past, and learning from their experiences.
Another possible theme is redemption. The story may show characters who have made mistakes in the past and are now on a path to make amends, to become the best version of themselves, which is also hinted at in the title 'Best of Me'.